A drill operator often wishes to deviate a wellbore or control its direction to a given point within a producing formation. This operation is known as directional drilling. One example of this is for a water, injection well in an oil field, which is generally positioned at the edges of thee field and at a low point in that field (or formation).
In addition to controlling the required drilling direction, the formation through which a wellbore is drilled exerts a variable force on the drill string at all times. Tins along with the particular configuration of the drill can cause the drill bit to wander up, down, right or left. The industrial term given to this effect is “bit-walk” and many methods to control or re-direct “bit-walk” have been tried in the industry. The effect of bit walk in a vertical hole can be controlled, by varying the torque and weight on the bit while drilling a vertical hole. However, in a highly inclined or horizontal well, bit-walk becomes a major problem.
At present, in order to deviate a hole left or right, the driller can choose from a series of special downhole tools such as downhole motors, so-called “bent subs” and more recently rotary steerable tools.
A bent sub is a short tubular that has a slight bend to one side, is attached to the drill string, followed by a survey instrument, of which an MWD tool (Measurement While Drilling which passes wellbore directional information to the surface) is one generic type, followed by a downhole motor attached to the drill bit. The drill string is lowered into the wellbore and rotated until the MWD tool indicates that the leading edge of the drill bit is facing in the desired direction. Weight is applied to the bit through the drill collars. And, by pumping drilling fluid through the drill string, the downhole motor rotates the bit.
U.S. Pat. No. 3,561,549 relates to a device, which gives sufficient control to deviate and start an inclined hole from or control bit-walk in a vertical wellbore. The drilling tool has a non-rotating sleeve with a plurality of fins (or wedges) on one side is placed immediately below a downhole motor in turn attached to a bit.
U.S. Pat. No. 4,220,213 relates to a device, which comprises a weighted mandrel. The tool is designed to take advantage of gravity because the heavy side of the mandrel will seek the low-side of the hole. The low side of the wellbore is defined as the side farthest away from the vertical.
U.S. Pat. No. 4,638,873 relates to a tool, which has a spring-loaded shoe and a weighted heavy side, which can accommodate a gauge insert held in place by a retaining bolt.
U.S. Pat. No. 5,220,963 discloses an apparatus having an inner rotating mandrel housed in three non-rotating elements.
Thus, it is known how to correct a bit-walk in a wellbore. However, if changes in the forces that cause bit-walk occur while drilling, all the prior art tools must be withdrawn in order to correct the direction of the wellbore. The absolute requirement for tool withdrawal means that a round trip must be performed. This results in a compromise of safety and a large expenditure of time and money.
U.S. Pat. No. 5,979,570 (also WO 96/31679) partially address the problem of bit-walk in an inclined wellbore. The device described in this patent application and patent comprises eccentrically bored inner and outer sleeves. The outer sleeve being freely moveable so that it can seek the low side of the wellbore, the weighted side of the inner eccentric sleeve being capable of being positioned either on the right side or the left side of the weighted portion of the outer eccentric sleeve to correct in a binary manner for bit walk.
U.S. Pat. No. 6,808,027 (one of the co-inventors of which is a co-inventor of the instant application) discloses an improved downhole tool which can correct for bit walk in a highly inclined wellbore and which is capable of controlling both the inclination and the azimuthal plane of the well bore. Whereas U.S. Pat. No. 5,979,570 discloses bit offset, the '027 patent discloses a vector approach (the actual improvement) called bit point. The '027 patent uses a series of sleeves (or cams depending on the definition of the term) that may be eccentric or concentric to obtain bit point (the improvement) or bit offset disclosed in the earlier patent, but obtained by a different mechanical device).
The instant application discloses a different mechanical technique to obtain the rotary vector within the downhole tool and may be employed in the apparatus of U.S. Pat. No. 6,808,027, U.S. Pat. No. 5,979,570 and other downhole equipment (using stabilizers, blades and the like) that require an internal positioning mechanism.